Apparently, people like to say things like “I’d only ever vote for X with a gun to my head” in order to test the canvasser.
If you run away, as the safe-seated subject of the conversation did, then they think you’re not worthy, and will be true to what they’ve said.
But if you stop and ask “why?” and then debate it with them, they’re actually likely to vote for you seeing as you’ve passed their little test.
Oh, and voters can tell when you’re outside deciding which person should talk to which household based on previous experience. They find it demeaning – best to drop the voter ID-ing act and just go to every door.
UPDATE: Of course, there’s an exception to this when you want to be able to get out the vote at short notice. i.e. on election day.